Header injection relies on being able to insert additional newlines into header variables, which makes the string look like a new header.
For example, allowing a subject value of Testing\nCc: firstname.lastname@example.org\n\nSome body text would result in a message header containing:
Some body text
i.e. the abuser has not only added additional recipients, but they've managed to supply their own body text too.
However in your case the $toaddress is constant, and even if $toaddress had been user-supplied it should be correctly sanitised by the mail() function.
Your subject header is similarly constant
The $message variable is safe because by definition that's the body text and only sent after the real headers.
That only leaves $fromaddress, and you're already using FILTER_VALIDATE_EMAIL on that which should also reject anything with a newline in it.
However you should strictly be checking the result of that test, and aborting the whole thing if the result is FALSE. As it is if the validation fails then mail() will complain about being given a blank From: address, but there's no header injection opportunity there.
As far as I can tell, then, this code is actually secure.
Also, IMHO, you shouldn't send the emails from the user-supplied email address. That would fall foul of anti-spam mechanisms such as SPF.
You should use a constant From: value belonging to your own domain. If you like you could then use a correctly sanitised value in the Reply-To header to make it easier to have the subsequent reply go to the desired address.